Field of the Invention
The present invention relates to dry denitration by ammonia of exhaust gas containing an arsenic component in the presence of a denitrating catalyst. More particularly, it pertains to a method for denitration that is capable of preventing the catalyst used from deteriorating, and to a method and an apparatus for regenerating a denitrating catalyst that is already degraded by the arsenic component of the exhaust gas being treated.
As a method for reducing NOx in exhaust gas from boilers, the dry denitration method using ammonia as a reducing agent is commonly employed. Since catalysts used in this method are active at gas temperatures between 200.degree. and 450.degree. C., a denitration reactor is commonly disposed between an outlet of a boiler fuel economizer and an inlet of an air preheating apparatus. In the following, a typical flow in a basic denitration method of this kind will be explained with reference to FIG. 1.
In FIG. 1, exhaust gas coming out of a fuel economizer 6 of a boiler 1 is sent to a denitration reactor 3 disposed in the upstream position of an air preheater via an exhaust gas duct 7 and is denitrated there. Ammonia, which is a reducing agent, is supplied from an ammonia supply apparatus 2 that is disposed on the exhaust gas duct in the upstream position of the denitration reactor. NOx in the exhaust gas is decomposed into water and nitrogen by the ammonia thus supplied and the denitration catalyst filled in the denitration reactor 3.
The exhaust gas, having been denitrated in the denitration reactor 3, goes through the air preheater 4 and the exhaust gas duct 7 and is removed of dust in a dust collector 5 and sent to the next process such as wet desulfurization.
Also, as shown in FIG. 3, it is possible to send the air for combustion into the boiler 1 by a forced draught fan (called F.D.F. hereinafter) 9 through an air duct 8 after preheating it in the air preheater 4.
When the denitration reactor 3 is attached to the exhaust gas duct 7 between an outlet of the boiler fuel economizer 6 and an inlet of the air preheater 4, if the fuel contains arsenic compounds, the exhaust gas at an inlet of the denitration reactor 3 also contains arsenic compounds. These gaseous arsenic compounds accumulate on the denitrating catalyst filled in the denitration reactor 3 and degrade the performance of the catalyst in a rather short period of time. This degradation of performance of the catalyst cannot be avoided under normal operating conditions of the denitration reactor 3. Therefore, replacement of the catalyst or the like is necessary to avoid lower performances, and this causes cost problems.
Further, in order to avoid the influence of arsenic compounds, the possibility of sending the exhaust gas from an outlet of a wet desulfurization apparatus, after reheating it, to a denitration apparatus has been considered. However, this possibility makes a reheating device and the fuel for reheating necessary. They, as well as the replacement of the catalyst described above, lead to a quite significant increase in costs.